While the direct production of hydrogen peroxide (H2O2) from hydrogen (H2) and oxygen (O2) is known in the art, commercial processes are typically indirect processes using a hydrogen donor organic compound as the source of hydrogen needed to react with oxygen in order to circumvent the explosive hazard of direct mixtures of hydrogen and oxygen. Usually, anthraquinone or a derivative thereof is employed as a hydrogen donor molecule by first reducing the molecule to the dihydro moiety and then oxidizing the reduced dihydro moiety with oxygen to yield hydrogen peroxide and the starting anthraquinone. While a relatively safe process, the indirect process has many drawbacks, not the least of which is the fact that it is a multistep process which consumes anthraquinone and solvent by oxidation.
The direct catalytic production of hydrogen peroxide from hydrogen and oxygen, although well-studied, has not achieved commercial acceptance as yet. When the direct process is carried out at hydrogen levels below 5% by volume to avoid the explosive hydrogen gas mixture range, the yields of hydrogen peroxide are low. Further, the process selectivity is low as a consequence of the conversion of hydrogen peroxide to water in the catalytic environment. The cost of hydrogen and oxygen is an important economic factor in the direct synthesis process. Inefficiencies in their use caused by low selectivity constitute a significant problem.
Another significant economic problem in direct hydrogen peroxide production arises from the use of large gas excesses. It is common practice in direct synthesis processes to employ large excesses of one of the gaseous components, especially oxygen. Consequently, large gas flows must be handled in the process. Since direct synthesis processes typically operate at pressures of at least 500 psig, and often greater than 1000 psig, the copious amount of excess oxygen in the reaction mixture which must be recompressed for recycle imposes a significant cost burden on the process. Large and expensive compressors are required to accommodate the recycle stream from direct synthesis processes that employ excessively large oxygen flows.
It is also well known in the prior art that the ratio of oxygen and hydrogen gases in the direct catalytic synthesis process has a critical effect on the yield of hydrogen peroxide produced as well as the selectivity of the process for hydrogen peroxide production. U.S. Pat. No. 4,336,239 teaches a direct synthesis hydrogen peroxide production process using noble metal catalysts where the molar ratio of oxygen to hydrogen is greater than about 3.4, preferably above 5 and most preferably a molar ratio of 12–15, at catalyst loadings of more than 30 mg per 100 ml of medium. According to the '239 patent, higher oxygen to hydrogen ratios above 3.4 results in an increase in the amount of hydrogen peroxide obtained.
U.S. Pat. No. 6,375,920 teaches a reactor system for hydrogen peroxide production wherein hydrogen is fed to the reactor in staged points of entry above an oxygen and hydrogen inlet. The process is distinguished by employing a woven catalyst having a long on-stream life in a fixed bed reactor which produces a selectivity of above 65%. The patent does not teach or claim the adjustment of the oxygen to hydrogen gas ratio at each stage to provide a preferred ratio that yields a minimum volume of a recycle stream
U.S. Pat. No. 6,447,743 teaches a method for preparing hydrogen peroxide directly using staged oxygen addition into the reactor at a relatively high ratio of oxygen to hydrogen.
U.S. Pat. No. 5,641,467 to Huckins teaches and claims a method for safe hydrogen peroxide production in a catalytic reactor by injecting oxygen or oxygen and hydrogen into a flowing medium at multiple points downstream in a catalytic reactor. The volumetric ratios of flowing medium to injected hydrogen and/or oxygen are selected to preferable maintain a safe combination of hydrogen to oxygen or where the volume ratio of oxygen to hydrogen is from 1:1 to 20:1. However, the patent does not teach or claim the staged injection of hydrogen at varying hydrogen to oxygen ratios preselected to maximize the conversion and selectivity of hydrogen peroxide production while producing low oxygen recycle ratios.
U.S. Pat. No. 6,042,804 is related to the foregoing '467 patent and teaches and claims separation of hydrogen peroxide plus process operating conditions within the explosive limits of hydrogen gas and oxygen mixtures.
It is an objective of the present invention to provide a process for the direct continuous synthesis of hydrogen peroxide from hydrogen gas and oxygen in a catalytic reactor that avoids the necessity of feeding a large excess of oxygen that results in a substantial recycle stream of unconverted oxygen. It is a further objective of the invention to provide such a process that avoids the production of and need for recycling a large excess of oxygen but, nevertheless, achieves improved process conversion of hydrogen and oxygen to hydrogen peroxide with high selectivity.